Acute injuries to smaller blood vessels and subsequent dysfunction of the tissue in which the injured blood vessels are located (microvascular angiopathies) are a common feature of the pathology of a variety of diseases of various organs, such as kidney, heart, and lungs. The injury is often associated with endothelial cell injury or death and the presence of products of coagulation or thrombosis. The agent of injury may, for example, be a toxin, an immune factor, an infectious agent, a metabolic or physiological stress, or a component of the humoral or cellular immune system, or may be as of yet unidentified. A subgroup of such diseases is unified by the presence of thrombotic microangiopathies (TMA), and is characterized clinically by non-immune hemolytic anemia, thrombocytopenia, and/or renal failure. The most common cause of TMA is the hemolytic uremic syndrome (HUS), a disease that more frequently occurs in childhood, where it is the most common cause of acute renal failure, but also affects adults where more severe clinical course is often observed. Although the pathogenesis of HUS has not been fully elucidated, it is widely accepted that the majority of these cases are associated with enteric infection with the verotoxin producing strain, E. coli O157. Verotoxins produced by E. coli induce glomerular endothelial cell (GEN) injury and generate renal thrombotic microangiopathy in most cases of epidemic HUS (Boyce et al., N. Engl. J. Med. 333:364–368 (1995)). Some patients, especially adults, may have a relative lack of renal involvement and are sometimes classified as having thrombotic thrombocytopenic purpura (TTP). However, thrombotic microangiopathies may also occur as a complication of pregnancy (eclampsia), with malignant hypertension following radiation to the kidney, after transplantation (often secondary to cyclosporine or FK506 treatment), with cancer chemotherapies (especially mitomycin C), with certain infections (e.g., Shigella or HIV), in association with systemic lupus or the antiphospholipid syndrome, or may be idiopathic or familial. Experimental data suggest that endothelial cell injury is a common feature in the pathogenesis of HUS/TTP. See, e.g. Kaplan et al., Pediatr. Nephrol. 4:276 (1990). Endothelial cell injury triggers a cascade of subsequent events, including local intravascular coagulation, fibrin deposition, and platelet activation and aggregation. The mechanisms that mediate these events are not well understood. In the case of verotoxin-mediated HUS, injury to the endothelium leads to detachment and death, with local platelet activation and consumption, fibrin deposition and microangiopathic hemolysis.
The renal corpuscule, commonly referred to as glomerulus, is composed of a capillary network lined by a thin layer of fenestrated endothelium; a central region of mesangial cells with surrounding mesangial matrix; the visceral epithelial cells and the associated basement membrane; and the parietal layer of Bowman capsule with its basement membrane. Between the two epithelial layers, there is a narrow cavity called the urinary space. The glomerulus is responsible for the production of an ultrafiltrate of plasma. The endothelial cells form the initial barrier to the passage of blood constituents from the capillary lumen to the urinary space. Under normal conditions, the formed constituents of the blood, such as erythrocytes, leukocytes, and platelets, do not gain access to the subendothelial space. In addition, because of their negative surface charge, the endothelial cells contribute to the charge-specific properties of the glomerular capillary wall. In the kidney, the damage to the glomerular and peritubular capillaries and arterioles results in ischemia and acute tubular necrosis, and, if severe, may lead to patchy or regional cortical necrosis. For further details see also Brenner & Rector's: The Kidney, Fifth Ed., Barry M. Brenner ed., W.B. Saunders Co., 1996.
The current treatment of HUS in children consists primarily of supportive therapy (dialysis, transfusions and attention to fluid and electrolyte balance). However, in adults and in refractory cases in children the addition of plasma infusion and/or plasma exchange therapy is also performed. Remuzzi and Ruggenenti, Kidney Int. 47:2–19 (1995). Data to support plasma exchange therapy is not conclusive, but uncontrolled trials have suggested a potential benefit, especially in terms of improving the thrombocytopenia, anemia, and associated neurologic signs (which consist of confusion, paresthesias, and occasionally coma). Most patients recover from the acute episode, although mortality rates of 3–8% are occasionally reported. Brandt and Avner, Hemolytic uremic syndrome and thrombotic thrombocytopenia purpura. In: Neilson and Couser, eds., Immunologic Renal Diseases, Lippincott-Raven, Philadelphia, 1996, pp. 1161–1181. However, some patients do not recover their renal function fully, and between 20 and 40% of patients will develop some degree of renal impairment or hypertension within 10–15 years, with as many as half progressing to dialysis. Brandt and Avner, supra. In 1995, HUS accounted for 2.4% of patients on dialysis. Patients at risk were those with greater than 50% glomerular involvement, arteriolar disease, or cortical necrosis. Habib et al., Adv. Nephrology 11:99–128 (1982).
There is a great need for new therapeutic agents for the treatment of microvascular angiopathies, and in particular, thrombotic microangiopathies (TMA). There is a particular need to find a way to preserve cells and maintain normal function of organs within which the blood vessels are undergoing, or have undergone, injury. Currently, no therapy has been proposed for the treatment of microvascular angiopathies that is targeted at preventing or reducing endothelial cell injury and stimulating the repair of injured endothelial cells. Indeed, most of the agents in clinical use are either aimed at removing or infusing unknown factors (plasma exchange/plasma infusion), inhibiting platelet action (antiplatelet drugs), or blocking the immune system (steroids and vincristine).
There is further a need for new approaches to the treatment of renal diseases involving injury to the glomerular endothelium and the tissues surrounding the injured glomerular blood vessels, and in particular, the treatment of hemolytic uremic syndrome (HUS).